Merge pull request #211239 from dschrempf/haskell-docs

# Haskell {#haskell}

The Haskell infrastructure in nixpkgs has two main purposes: The primary purpose

The Haskell infrastructure in Nixpkgs has two main purposes: The primary purpose

is to provide a Haskell compiler and build tools as well as infrastructure for

packaging Haskell-based packages.

The secondary purpose is to provide support for Haskell development environment

The secondary purpose is to provide support for Haskell development environments

including prebuilt Haskell libraries. However, in this area sacrifices have been

made due to self-imposed restrictions in nixpkgs, to lessen the maintenance

effort and improve performance. (More details in the subsection

made due to self-imposed restrictions in Nixpkgs, to lessen the maintenance

effort and to improve performance. (More details in the subsection

[Limitations.](#haskell-limitations))

## Available packages {#haskell-available-packages}

* Language specific tools: `cabal-install`, `stack`, `hpack`, …

Many “normal” user facing packages written in Haskell, like `niv` or `cachix`,

are also exposed at the top level, so there is nothing haskell specific to

are also exposed at the top level, and there is nothing Haskell specific to

installing and using them.

All of these packages originally are defined in the `haskellPackages` package

All of these packages are originally defined in the `haskellPackages` package

set and are re-exposed with a reduced dependency closure for convenience.

(see `justStaticExecutables` below)

The `haskellPackages` set includes at least one version of every package from

Hackage as well as some manually injected packages. This amounts to a lot of

packages, so it is hidden from `nix-env -qa` by default for performance reasons.

You can still list all packages in the set like this, though:

You can still list all packages in the set like this:

```console

$ nix-env -f '<nixpkgs>' -qaP -A haskellPackages

haskellPackages.abc-puzzle                                                  abc-puzzle-0.2.1

…

```

Also the default set `haskellPackages` is included on [search.nixos.org].

Also, the `haskellPackages` set is included on [search.nixos.org].

The attribute names in `haskellPackages` always correspond with their name on

Hackage. Since Hackage allows names that are not valid Nix without extra

escaping, you sometimes need to extra care when handling attribute names like

`3dmodels`.

Hackage. Since Hackage allows names that are not valid Nix without escaping,

you need to take care when handling attribute names like `3dmodels`.

For packages that are part of [Stackage], we use the version prescribed by a

Stackage solver (usually the current LTS one) as the default version. For all

other packages we use the latest version from Hackage. See

[below](#haskell-available-versions) to learn which versions exactly are provided.

[below](#haskell-available-versions) to learn which versions are provided

exactly.

Roughly half of the 16K packages contained in `haskellPackages` don't actually

build and are marked as broken semi-automatically. Most of those packages are

deprecated or unmaintained, but sometimes packages that should, don't build.

Very often fixing them is not a lot of work.

deprecated or unmaintained, but sometimes packages that should build, do not

build. Very often fixing them is not a lot of work.

<!--

TODO(@sternenseemann):

haskell.compiler.ghcjs                   ghcjs-8.10.7

```

Every of those compilers has a corresponding attribute set built completely

using it. However, the non-standard package sets are not tested regularly and

have less working packages as a result. The corresponding package set for GHC

9.4.4 is `haskell.packages.ghc944` (in fact `haskellPackages` is just an alias

for `haskell.packages.ghc924`):

Each of those compiler versions has a corresponding attribute set built using

it. However, the non-standard package sets are not tested regularly and, as a

result, contain fewer working packages. The corresponding package set for GHC

9.4.4 is `haskell.packages.ghc944`. In fact `haskellPackages` is just an alias

for `haskell.packages.ghc924`:

```console

$ nix-env -f '<nixpkgs>' -qaP -A haskell.packages.ghc924